Radioactive labeling of proteins using tritium is a very important technique used in neurological research to identify, isolate, and investigate the expression of proteins in biological systems. It is often necessary to use tritium as a radiolabel for this work because 3H in vivo labeled proteins retain their conformal structure and are suitable for binding assays. Currently, many 3H-labeled binding assays are done using inconvenient, time consuming, and expensive techniques involving scintillation cocktail measurement because no reliable and sensitive tritium detector is available to quantify the radioactivity directly and non-destructively from the sample. We propose to develop a novel solid-state based detector system capable of measuring the spatial activity distribution of tritiated biological specimens without the use of labor intensive liquid scintillation fluid techniques. This system is based on a large area array of silicon avalanche photodiodes (APDs) which will be capable of directly sensing the low energy radiation emissions of the 3H-labeled samples collected on filter paper. A system based on the parallel acquisition of many such pixil elements would provide superior sample throughput for a wide variety of 3H-binding assays and thus will represent a significant productivity increase for many procedures while greatly reducing the quantity of chemical and radioactive waste that is generated. PROPOSED COMMERCIAL APPLICATION: Many biomedical facilities now use very expensive counting systems which run continuously to keep up with the constant flow of samples to be measured. A new instrument would provide improved cost effectiveness and greatly reduce the problems associated with hazardous waste. This would find a good commercial market.